纯虚函数（pure virtual function ）和抽象类(abstract base class)

函数体=0的虚函数称为“纯虚函数”。包含纯虚函数的类称为“抽象类”

#include <string> class Animal // This Animal is an abstract base class 
{
protected:
    std::string m_name;

public:
    Animal(std::string name)
        : m_name(name)
    {    }

    std::string getName() { return m_name; }
    virtual const char* speak() = 0; // note that speak is now a pure virtual function     
};

int main() {
    Animal a; //错：抽象类不能实例化(不能定义抽象类的对象(变量))     
}

//从抽象类派生的类型如果没有继承实现所有的纯虚函数，则仍然是“抽象类” 
#include <iostream> class Cow : public Animal
{
public:
    Cow(std::string name)
        : Animal(name)
    {
    }

    // We forgot to redefine speak     
};

int main(){
    Cow cow("Betsy"); //仍然错：因为Cow仍然是抽象类     
    std::cout << cow.getName() << " says " << cow.speak() << '\n';
}

像下面这样实现所有纯虚函数就没问题了，Cow不是一个抽象类

#include <iostream> 
class Cow : public Animal
{
public:
    Cow(std::string name)
        : Animal(name)
    {
    }

    virtual const char* speak() { return "Moo"; }
};

int main()
{
    Cow cow("Betsy");
    std::cout << cow.getName() << " says " << cow.speak() << '\n';
}